New, alternative, forms of cross-border cooperation, in particular processes of informal international lawmaking (‘INLAW’), have emerged and gained prominence since the 2000s in response to an increasingly diverse, networked, and knowledge-based society. This transformation impacts on traditional notions of public international law as well as on the conceptualization of an emerging international rule of law. This is all the more the case given the tendency to emphasise the formal characteristics of the latter. The present article, nevertheless, challenges the assumption that informal international law is, because of its informal nature, by definition incompatible with a rule of law approach. The article thereto reintroduces a negative (shielding society from arbitrariness) and a positive (providing requirements for lawmaking) conception of the rule of law. First, it identifies actors and processes of informal international lawmaking and elucidates the extent of their reliance on legal principles as opposed to the wielding of arbitrary power on the international plane. Second, it operationalises a rule of law approach in the three phases of informal lawmaking processes (lawmaking per se, law-application and accountability). As such, it discusses how a number of concrete rule of law requirements related to participation, procedural guarantees, accessibility of norms, and accountability mechanisms are implemented. To conclude, it is submitted that rule of law theories can be reconciled with IN-LAW, without affecting the flexibility that is fundamental to the non-traditional norms produced at the global level.